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Alta combi recirc

riverdog
riverdog Member Posts: 20
in Domestic Hot Water

hello I have a Alta combo boiler installed and the homeowner wants hot water faster, with that said the house is finished and no way to get a dedicated return line for domestic, so my ? Is can you install a crossover system, let’s say a grunfos system, to the combi boiler and will it work, also there is 40 gal electric wh installed before the combi preheating water to 70 degrees, thanks

Comments

  • hot_rod
    hot_rod Member Posts: 25,650
    edited September 16

    The combis with internal mini buffer tanks work best for recirc so you aren't cycling the unit constantly. You need the dhw reciec flow to meet the tankless minimum.

    It seems the 40 tank after the combi would be a better option. Is the electric tank powered? Or just warming to ambient?

    An example of the Navian buffer/ recirc approach.

    Rinnai has commercial tankles with tanks built in, go figure.

    Also an example of the Rinnai option for tanked storage and pumped tankless.

    https://www.youtube.com/watch?v=Ep2vwFmpmfI
    Screenshot 2025-09-16 at 7.58.41 AM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • riverdog
    riverdog Member Posts: 20

    the electric tank is preheating the water to 70 degrees, sort of weird but the Home owner wants for gpm from combi so hotter water going in then unit, so I was going to put a domestic pump on hot side also times with crossover tee out in the living space, so the pump will go for 15 min and off for 45 my hope is that the thermo crossover valve will dead head pump and stop flow which should happen fast

  • hot_rod
    hot_rod Member Posts: 25,650

    so a tankless supplying heat and DHW?

    A combined system?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • riverdog
    riverdog Member Posts: 20

    no there is no tankless, there is a Combi boiler a Combi supplies heat to the house and also domestic hot water, there is a 40 gallon electric hot water heater heating the water up to 70° before it enters the boiler on the domestic side, the reason for this electric hot water tank before the domestic side is I live in Montana and the water coming out of the ground Here is very cold so you get more gallons per minute when it is preheated, until the 40 gallons of preheated wateris gone.

  • Kaos
    Kaos Member Posts: 713

    The water tank should be after the combi. Set the tank to run at lower temp than what the combi puts out, this way under normal use, the tank will never run but under high draws it will add and extra 40 gallons of hot water.

    If you want to get fancy, you can tie the combi and the tank together with a mixing valve (cold to combi, hot to water tank, mix to house). Set up like this the water tank will only see flow when the combi can't keep up and water temperature drops. This will stretch the water further as it only needs to add enough to get that bit of temperature boost.

    This gets you the same performance boost as the pre-heater without using electricity all the time plus it eliminates hot water delay when the combi is not heating.

    As for recirc, the bypass type work well just as long as the owner is ok with warmish water on the cold line some of the time. Demand type controls is the best for combi/tankless setups.

  • riverdog
    riverdog Member Posts: 20

    so I like what you’re saying about putting the on the cold side and the tank on the hot sideof the mixing valve plus you need a mixing valve on a Combiso it makes sense to put it there so in that situation would you put the recirculation pump with crossover valves on the outlet side of that mixing valve? I’m guessing that is the only place you can put it in that situation

  • hot_rod
    hot_rod Member Posts: 25,650

    There is a specific way you need to pipe a thermostatic valve with a recirc system to prevent temperature creep. Check valves and bypass particularly.

    Explained in Idronics 11

    IMG_1042.jpeg Screenshot 2025-09-18 at 8.04.06 PM.png

    https://www.caleffi.com/sites/default/files/media/file/Idronics_11.pdf

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Kaos
    Kaos Member Posts: 713

    https://forum.heatinghelp.com/discussion/comment/1865349#Comment_1865349

    This is not your tampering mix valve. That has to be a separate valve as it needs cold water to do its job.

    The mix valve here is just used as an automatic 3 way valve that selects where the hot water comes from based on temperature.

    Say you set the mix valve to 110F, combi to 120F and tank to 140F. As long as the combi is putting out 120F, the mix is switched all the way to the cold side and you get 120F water out. If the combi starts loosing steam and only puts out 90F water, the mix valve will now start using the 140F water from the tank to mix it up to put out the 110F setpoint.

    Since you are using a thermostatic bypass for recirc, the return water is warmish, so the hot water creep @hot_rod mentioned is not usually an issue.

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